The present invention relates to a process for preparing methionine aminoamide. It also relates to the preparation of methionine without coproduction of sales, from an aqueous solution containing essentially methionine aminonitrile (or 2-amino-4-methylthiobutyronitrile, termed AMTBN), through the production, during a first step, of aminoamide, followed by two further steps.
The process for preparing methionine aminoamide (or 2-amino-4-methylthiobutyramide, termed ATMBM) consists in hydrating methionine aminonitrile in the presence of a catalyst of ketone type and of a basic resin of OH type. This hydration allows the production of a solution containing methionine aminoamide completely free of inorganic salts.
The process for preparing methionine consists of a process in three steps, the first of which is the step described above:
the aminonitrile is hydrated to methionine aminoamide in the presence of a catalyst of ketone type and of a basic resin of OH type,
the methionine aminoamide is hydrolyzed to ammonium methioninate via various possible pathways:
via the chemical pathway by homogeneous catalysis, such as for example hydrolysis with aqueous ammonia,
via the chemical pathway by heterogeneous catalysis, such as for example oxide-catalyzed hydrolysis,
via the enzymatic pathway,
finally, the methionine is recovered from the ammonium methioninate, after entrainment of the ammonia by stripping.
It is known, for example according to patent EP 84470 or EP 168282, that hydration of aminonitrile can be carried out on polymeric resin grafted with groups of ketone type (xe2x80x94(Cxe2x95x90O)xe2x80x94), in the presence of hydroxide ions. The implementation of such a process, using a strong alkaline base (sodium hydroxide, potassium hydroxide, etc.) to supply the hydroxide ions required for catalysis, leads to the production of an aminoamide contaminated with the corresponding alkali.
In addition, it is known practice, for example according to patent EP 228938, to prepare methionine from methionine aminoamide using a strong base (for example, sodium hydroxide or another alkaline base) in order to carry out the alkaline hydrolysis of the amide. The implementation of such a process leads, during the acidification of the alkaline methioninate produced with a strong inorganic acid (sulfuric or hydrochloric acid), to the coproduction of an inorganic salt: the corresponding alkali metal sulfate or chloride.
In the two processes mentioned above, the use of a strong alkaline base as a catalyst for hydrating the aminonitrile and/or as a reagent for hydrolyzing the amide, leads to the coproduction of an inorganic salt: the corresponding alkali metal sulfate or chloride, which must then be separated from the methionine, often with great difficulty, using expensive processes of crystallization, filtration or successive concentration of the mother liquors.
The process according to the invention makes it possible to avoid the use of an alkaline base and, therefore, the coexistence, within the same flow, of methionine and of the inorganic salt, which must then be separated as described above.
Thus, the process according to the invention makes it possible to produce a flow of methionine free of inorganic salt.
The methionine aminonitrile (or 2-amino-4-methylthiobutyronitrile) can be produced by reacting aqueous ammonia on the corresponding cyanohydrin or any other means which would not bring salt into the medium.
Finally, the process according to the invention, for preparing the amide on hydroxide resin, makes it possible, during the contact with this resin, to eliminate the residual cyanides/nitriles which may exist at the end of the aminonitrile synthesis and which are harmful in the case of enzymatic hydrolysis of the aminoamide to ammonium methioninate, and also the presence of which is to be avoided in the final product.
For the catalyzed hydration of the aminonitrile to aminoamide, the resins are in particular chosen from basic hydroxide resins, such as the commercial resins sold under the trade names ROHM and HALS Ambersep 900 OH, or FLUKA.
After prolonged use, the resin is optionally regenerated by treatment in sodium-containing medium. The regeneration is preferably carried out with a sodium hydroxide solution according to a weight concentration of NaOH of at least 4% and according to an amount of NaOH of 80 to 150 g/liter of resin.
The present invention also relates to an industrial process for preparing methionine. According to this process, methylthiopropionic aldehyde is reacted with hydrocyanic acid in the presence of a basic catalyst or of a buffer allowing the pH of the solution to be maintained at between 5.5 and 7.5; among these catalysts, mention may be made of: tertiary amines, in particular triethylamine or pyridine, and citrate buffer. The hydrocyanic acid is used either pure or in a mixture with the gases derived from the synthesis thereof, such as nitrogen, carbon dioxide, carbon monoxide, water and methane, after, in particular, eliminating the ammonia. The reaction is preferably carried out at room temperature and in the presence of equimolar amounts of each of the reagents or of amounts of hydrocyanic acid which are slightly above the stoichiometry, such as, for example, according to an excess with respect to the stoichiometry of approximately 5%. The reaction can take place in a stirred reactor or in a tubular reactor; it can also take place in a gas-liquid contactor so as to allow reactive absorption, this being preferably when gaseous hydrocyanic acid is used.
At the end of the cyanation step, an aqueous solution of 2-hydroxy-4-methylthiobutyronitriles termed HMTBN (101), is obtained. This solution is brought into contact with ammonia (102) or with an ammonia/water mixture. The molar amount of ammonia used related to the HMTBN is advantageously between 4 and 7. The amounts specified hereinabove are described in patent DE 2645544. The ammonia is used pure or in aqueous solution. The aqueous solution of ammonia is preferably used at a concentration greater than 25% by weight, and preferably at a concentration greater than 60% by weight. Pure ammonia is most preferably used. The reaction temperature is preferably between 40 and 80xc2x0 C., and most particularly between 55 and 70xc2x0 C. The reaction is, in particular, carried out in a stirred or tubular flow reactor (A), in particular a plug-flow reactor. At the end of the reaction, an aqueous solution of 2-amino-4-methylthiobutyronitrile (106) is obtained.
At the end of the reaction, a portion of the excess ammonia is eliminated or separated by expansion of the simple flashtype or with entrainment using an inert gas such as, for example, nitrogen or steam, in a column (B). The temperature of the medium, during the separation step, is less than 60xc2x0 C., and preferably between 15 and 50xc2x0 C. The ammonia recovered (109), at the top of the column (B), is then absorbed in an absorber (C). The aqueous ammonia solution obtained (415/422) is then distilled (D). The gaseous ammonia obtained at the head of the distillation column (423) is condensed (E), and then, after optionally adding ammonia (426), is preferably reinjected in the form of a stream (427) into the aminonitrile synthesis reactor. The recycled ammonia stream (427) which is introduced into the aminonitrile synthesis reactor contains preferably less than 5% by weight of acetone.
The bottom (424) of the distillation column (D), containing water, acetone and ammonia, is divided into two streams (205) and (410). The lesser stream (205) is returned to the aminoamide synthesis and the greater stream (410) is sent to the absorber (C).
The aminonitrile contained in the stream (110) which is obtained at the bottom of the column (B) and which still contains in particular from 0.1 mol to 1.5 mol of residual ammonia per mole of aminonitrile is hydrated in the presence of a ketone (202 and 205), by contact with an ion-exchange resin of hydroxide type, in the reactor (F).
Among ketones, acetone is preferably used. The hydration of the aminonitrile is carried out in the presence of 0.1 to 1, preferably 0.2 to 0.6, acetone equivalents/mole of AMTBN. The hydration is carried out in particular in the presence of an amount of basic resin such that the number of hydroxide equivalents is, in particular, between 0.10 and 1, and preferably between 0.15 and 0.5, equivalent/mole of AMTBN. The reaction temperature is preferably set between 10 and 40xc2x0 C., and even more preferably between 15 and 25xc2x0 C. The reaction can be carried out continuously, discontinuously or semicontinuously, in a stirred or tubular reactor-type system or, finally, in a column containing the basic resin.
At the end of the reaction, an aqueous solution (208) containing the methionine aminoamide, aqueous ammonia, acetone, water and various organic compounds, such as, by way of nonlimiting indication, methionine or imidazolidinone, is obtained.
The mixture (208) obtained is, in particular, distilled in the column (G). The top of the column (211) is condensed in the condenser (H) so as to recover acetone containing little ammonia (212). This stream is recycled to the aminoamide synthesis. The uncondensable stream (213), containing essentially ammonia, is sent to the absorber (C).
The aqueous mixture (214) remaining in the bottom of the distillation column after elimination of the acetone, containing essentially the methionine aminoamide and the various organic compounds is then diluted (303) and then directed toward the aminoamide hydrolysis section (I).
This hydrolysis can be carried out via one of the following 3 pathways:
via the chemical pathway by homogeneous catalysis: hydrolysis with aqueous ammonia
For better implementation of the invention, an NH3/AMTBM molar ratio of between 2 and 25 mol/mole is preferably used. An AMTBM substrate concentration of preferably between 0.05 and 1 mol/kg is also used. With regard to the conditions of implementation of the invention, a temperature of between 100 and 180xc2x0 C. is preferably used.
via the chemical pathway by heterogeneous catalysis
This hydrolysis is catalyzed, in particular, using one of the following oxides: TiO2, TiO2/Al2O3, Nb2O5, Nb2O5xe2x80x94Al2O3, ZnO and ZrO2, and the heteropolyacids. The reaction is carried out preferably at the boiling point of the reaction mixture (approximately 100xc2x0 C.), under atmospheric pressure and under continuous flushing of nitrogen so as to eliminate the ammonia formed. The AMTBM substrate concentration in the reaction medium is, according to a better embodiment of the invention, between 0.1 and 2 mol/kg and the catalyst/substrate mass ratio is between 0.5 and 1.5.
The latter pathway is particularly advantageous with respect to the processes described in patents JP 03093753, JP 03093754, JP 03093755 and JP 03093756 since:
it makes it possible to carry out the reaction at lower temperature and pressure, which makes it possible to avoid heat decomposition of the reagents and products,
it makes it possible to recover acid methionine directly (while hot), without any other form of acidification and, therefore, without consuming inorganic acid. The separation of the solid catalyst and of the solubilized methionine while hot is easy.
via the enzymatic pathway using an amidase
This amidase is chosen from Rhodococcus IBN 20 or Brevibacterium R312 amidases. It is possible to use the genetic information encoding the amidase and to express it in a host microorganism. The host microorganism is, in particular, chosen from Escherichia coli or a member of the Corynebacterium genus. This biological material expressing the amidase activity is preferably immobilized.
The stream derived from the amide hydrolysis (305) is then treated, in the column (J), by entrainment using an inert gas (306), as chosen for example from nitrogen or steam, in a column, the temperature of which is regulated by the inlet streams of the various gases. In this column, the pressure is maintained preferably between 105 and 2xc3x97105 Pa. At the top of the column (J), the ammonia and a portion of the water (308) are recovered, which are sent to the absorber (C).
The bottom of the column (J) containing an aqueous solution of methionine (311), free of ammonia, is concentrated (K) until a very concentrated slurry is obtained which can be vehicled to filtration or any other separation system (L) where it can be completely dried to the pulverulent state.